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I also added a unit test for occlusion tracker pause during
overview transition.

Bug: None
Test: covered by unittests.

Change-Id: I5e8441e176eaa0dc9793d22059edb22e357d899e
Reviewed-on: https://chromium-review.googlesource.com/c/1344888
Commit-Queue: Mitsuru Oshima <oshima@chromium.org>
Reviewed-by: Sammie Quon <sammiequon@chromium.org>
Cr-Commit-Position: refs/heads/master@{#611323}